The present application relates to semiconductor integrated circuits, and more particularly to contact formation in SOI structures.
Semiconductor-on-insulator (SOI) structures have grown in use as devices are scaled down in size. SOI technology allows structures to be fabricated in the buried oxide (BOX) to which electrical contact can be made from the top of the device.
Contacts are typically fabricated by etching through pre-metal dielectric (PMD) material down to the object to which connection is to be made. If the connection is to be made to a buried gate or other structure inside the BOX, etching must go through the PMD and part of the BOX. If connection is to be made to structure above the BOX, etching need only go through the PMD and would preferably be stopped before penetrating the BOX.
Alignment is critical in making connections, and lithographic patterning size and accuracy limits the robustness of the connections. Misalignment of etching can cause the etchant to go too deep, even reaching the substrate.
Prior attempts at alleviating the narrow margin of error on contact etches have dealt with using a PMD liner after the moat etch, which requires a change of etch chemistry to etch through this liner. Additionally, contacts which partially overhang the contacted structure are less robust because of decreased area of contact, which also creates higher contact resistance.
The present application discloses an improved SOI structure for etching contacts to the moat. In one class of embodiments, the buried oxide comprises three layers: oxide, nitride, and oxide. The nitride layer acts as an etch stop relative to the oxide, since the two substances have different etching characteristics. This allows the contact to the moat to be made at the edge, or overlapping the edge. Normally, without adding etch stop layers and associated processing after moat etch, an overlapping contact etch would proceed past the moat into the buried oxide, and possibly through to the substrate. With an etch stop layer in the buried oxide, the contact etch can be made as close to the end of the moat as desired without fear of overetch, and without the complication of post-moat-etch etch stop layers. Variations on this idea are also presented, including (but not limited to) using fewer layers to create an etch stop, nitridating the buried oxide surface, and using an oxy-nitride instead of oxide as the buried insulating layer.
In another class of embodiments, a buried gate structure is fabricated in the buried oxide. This insulating region comprises oxide with a nitride layer, either below or adjacent to the buried gate.
Advantages of the disclosed methods and structures, in various embodiments, can include one or more of the following:
greater degree of freedom in contact alignment and/or design;
prevents overetch;
process simplicity relative to post-moat-etch etch stop techniques;
more robust contact, when contact overlaps into the isolation region;
option for etch stop for contacts to structures in the buried oxide.